The present invention relates to an electromagnetic clutch in which a shaft rotatable about an axis thereof is electrically connected to an input-side rotary member such as a gear which is rotatably mounted onto the shaft.
Heretofore, an electromagnetic clutch of the above type has been constructed, for example, as shown in FIG. 8, which is a cross-sectional view of the electromagnetic clutch. The electromagnetic clutch shown in FIG. 8 is generally used as a part operatively connected to a sheet feed mechanism of an image forming apparatus such as a copier. The electromagnetic clutch includes a field 51 of a blind-hole double-layered tubular structure having a tubular fitting portion 50 in the middle, a ring-like energizing coil 52 mounted on the fitting portion 50 in the field 51, and a shaft 53 inserted in the field 51 to be rotatable about an axis thereof. A rotor 55 formed with a circumferentially extending slit (through hole) 54 is fixed on the shaft 53 near the energizing coil 52.
A gear 56 is rotatably mounted onto the shaft 53 at a position opposing the energizing coil 52 with the rotor 55 interposed between the gear 56 and the energizing coil 52. The gear 56 is rotatable when a rotating force from an external device is transmitted thereto. An armature 57 is mounted on the shaft 53 opposing to the gear 56 with a plate spring 58 interposed between the armature 57 and the gear 56. Specifically, the armature 57 and the gear 56 are integrally assembled with the plate spring 58 in such a manner that the plate spring 58 is fixed to the armature 57 by swaging projections 59 which are integrally and circumferentially formed on a surface of the armature 57 with the plate spring 58 simultaneously mounted on the gear 56 by screws 60 at plural positions circumferentially.
Annular grooves 61, 62 are formed in opposite ends of the shaft 53. Fitting ring-like engaging pieces 63, 64 respectively in the annular grooves 61, 62 prevents the field 51 and the gear 56 from disengaging from the shaft 53. Note that at least the field 51, shaft 53, rotor 55, and armature 57 are made of a magnetized material such as ferrite metal.
In the above-constructed electromagnetic clutch, when the energizing coil 52 is excited, the armature 57 is magnetically attracted to the rotor 55 against a spring force of the plate spring 58, and thus, a rotation of the gear 56 is transmitted to the rotor 55 to rotate the shaft 53. On the other hand, when the energizing coil 52 is de-energized, the armature 57 springs away from the rotor 55 and returns to the original position opposing the gear 56 due to a restoration force of the plate spring 57. Thereby, the rotation of the shaft 53 is suspended.
In the electromagnetic clutch shown in FIG. 8, assembling operations such as tightly fixing the plate spring 58 onto the armature 57 by swaging or the like and fixing the plate spring 58 on the gear 56 with screws are required, which makes the assembling operation complex and may result in raising the production cost of the electromagnetic clutch with a difficulty in reducing the size of the plate spring 58. Further, it is highly likely that variation of a swaging force and a fixing force may occur at the swaging and fixing operation, which may resultantly vary the resilient performance of the plate spring 58. In view of the above possibility, swaging/fixing operation should be carried out with caution, which may deteriorate the assembling efficiency.
Another arrangement of the electromagnetic clutch has been proposed (for example, in Japanese Examined Utility Model Publication No. 7-24666) as shown in FIGS. 9A and 9B. FIG. 9A is a partially sectional view illustrating a state that an armature is mounted on a gear, and FIG. 9B is a plan view of a spring used in this electromagnetic clutch.
As shown in FIG. 9A, the electromagnetic clutch is constructed in such a manner that an armature 57' is mounted on a gear 56' together with a ring-like spring 58' formed with a through hole 582' in the center thereof. As shown in FIG. 9B, the ring-like spring 58' is formed with radially inwardly protruding projections 581' along an inner circumference thereof at a certain interval. Each projection 581' is formed with a bent portion 583' at a lead end thereof. Placing the armature 57' along a fitting groove 562' formed in a circumference of the gear 56' near a through hole 561' of the gear 56' and press-fitting the armature 57' into the fitting groove 562' resiliently holds the armature 57' in the gear 56'.
The electromagnetic clutch shown in FIGS. 9A and 9B (namely, the latter arrangement) can omit the swaging/fixing operation which has been required in the electromagnetic clutch shown in FIG. 8 and simplify the assembling operation to some extent which may contribute to lowering the production cost of the electromagnetic clutch to some extent. The latter arrangement may also be advantageous in improving assembling efficiency because merely press-fitting the ring-like spring 58' into the fitting groove 562' accomplishes mounting of the armature 57' onto the gear 56'. However, the latter arrangement necessitates an additional step of forming the bent portion 583' at the lead end of the projection 581' after press-molding the ring-like spring 58'. Further, the latter arrangement has a difficulty in producing a small-sized spring 58' due to the existence of the radially inwardly protruding projections 581', thereby likely to raise the production cost of the electromagnetic clutch as a whole.